1. Field of the Invention
The invention relates to methods for mounting thermoplastic substrates to organic, dielectric films and the resulting optical filters. More particularly, it relates to methodologies for fusing, through a variety of industrial processes, thermoplastic substrates to dielectric films without delamination while improving optical transmission at the film's refractive boundary.
2. Description of the Prior Art
Dielectric stacks have previously been constructed by sputtering layers of inorganic (metal) oxides onto polymeric substrates. The layers alternate between high and low indices of refraction, whereby incoming light encounters a partial mirror at each new layer. By creating so-called quarter wave stacks, certain wavelengths can be destructively interfered via Bragg diffraction. These constructs possess several advantages as follows: they are intimately, optically coupled to the substrate thereby providing high transmissions; they are custom engineered for the shape and curvature of each particular part. However, numerous disadvantages exist with these constructs as follows: the labor, materials and equipment costs for manufacturing are prohibitive; the mechanical coupling of metal oxide layers onto polymeric substrates is subject to delamination from flexural stress; the differing rates of thermal expansion between the metal oxides and the polymeric substrate can also cause delamination.
Recently, dielectric stacks have been manufactured from alternating layers of polymers having high and low indices of refraction, creating organic, optical, dielectric films. Several examples of these films are described in U.S. Pat. No. 5,882,774, entitled Optical Film, the contents of which are incorporated herein by reference thereto. The different polymers have similar mechanical properties and melting temperature profiles allowing hundreds of layers to be stacked and stretched to control the thickness and optically tune the film. While this allows the film to be economically manufactured, a problem exists in optically coupling the film to other components, for example, an optical thermoplastic substrate. Because the delicate film is highly susceptible to delamination, melting, burning or having its desirable transmission properties altered, previous attempts to mount the film have been limited to adhesion via optical fluids or optical adhesives. While these methods are adequate, they severely reduce the film's high transmission characteristics, by providing refractive boundaries from film-to-adhesive and from adhesive-to-substrate.
Accordingly, it would be desirable to mount the film onto a substrate with structural and mechanical integrity while maintaining, or enhancing, the film's transmission characteristics.